Blow-molding is a process for molding a preform part into a desired product. The preform is in the general shape of a tube with an opening at one end for the introduction of pressurized gas, typically air; however, other gases may be used. One specific type of blow-molding is stretch blow-molding (SBM). SBM can be used in a wide variety of applications; however, one of the most widely used applications is in the production of Polyethylene terephthalate (PET) products, such as drinking bottles.
SBM is typically performed with an SBM machine. FIG. 1 depicts an example SBM machine As may be seen from FIG. 1, example SBM machine 100 includes a blow valve block 102, a blow cylinder 104, and a blow mold 106. Blow valve block 102 provides low-pressure and high-pressure fluid supplies to blow cylinder 104, and may also be used to reclaim gas, or to exhaust gas.
The blow cylinder moves reciprocally between a retracted and extended position to come into contact with a preform 108 positioned below. The travel of the portion of the blow cylinder that makes contact with preform 108, preform 108 being positioned in blow mold 106, indicated by two-headed arrow 112, is typically on the scale of 25-35 mm. In the retracted position, blow cylinder 106 is positioned to allow a gripper (not shown) to insert preform 108 into blow mold 106 or remove a finished product from blow mold 106.
In FIG. 1, blow cylinder 104 is represented in an extended position, providing a fluid connection between blow valve block 102 and a preform 108. In the extended position, blow cylinder 104 seats on a portion of preform 108 external to blow mold 106 to create a seal. In FIG. 1, a stretch rod 110 is extended into preform 108. Stretch rod 110 may be used to longitudinally expand preform 108 into mold cavity 106. A portion of preform 108 is positioned internal to blow mold 106, and a portion of preform 108 remains external to blow mold 106. Blow mold 106 includes a cavity that comprises the outer shape of the desired product.
Typically, the SBM process uses a low-pressure fluid supply from blow valve block 102 along with stretch rod 110 during a pre-blow phase to stretch the preform 108 in a longitudinal direction and radially outward. The SBM process next uses a high-pressure fluid supply during a blowing phase to expand the portion of preform 108 into blow mold 106. The resulting product is generally hollow with an exterior shape conforming to the shape of mold cavity 106. The gas in the preform 108 is exhausted through blow valve block 102. This process is repeated during each blow-molding cycle.
Blow cylinder 104 provides a fluid connection between the blow valve block 102 and preform 108 in blow mold 106. The SBM machine 100 only applies pressurized gas to the preform 108 when blow cylinder 104 in an extended position. Blow cylinder 108 travel time can therefore add considerably to the overall SBM cycle time to fabricate a bottle or product. It is important to extend and retract blow cylinder 104 as quickly as possible, in instances cycle times between 40-60 ms are desirables. Simply increasing the acceleration of blow cylinder 104 can damage the SBM machine components, however. Blow cylinder 104, preform 108, blow mold 106, and any seals therein may be damaged if forcefully impacted. It is therefore important that blow cylinder 104 and/or any seal make gentle contact with preform 108 and/or blow mold 106.
Prior blow cylinders designs rely on hydraulic braking, industrial shocks, and spring or rubber buffers for damping. These prior damping technologies feature braking distances that can encompass a relatively large percentage of the typical 25-35 mm blow cylinder travel distance. When a large percentage of travel distance is required to brake a blow cylinder, the SBM cycle time is slower.
Existing blow cylinders are complicated, include many component parts, and have relatively large moving masses in the range of 800-2500 g. The larger moving masses require additional force and damping in an SBM process cycle.
The SBM process may further include hot fill process. In a hot fill process a PET bottle is blown in a heated blow mold 106. The heated mold may increase the degree of crystallization of the resulting bottle to produce, among other qualities, a heat-resistant bottle which can be used with hot beverages. The hot fill process often accompanies the pre-blow portion of the SBM process. Heat from the blow mold has the potential to damage the seals positioned between the blow cylinder 100 and blow mold 106 or preform 108 adjacent to blow mold 106, however. Therefore, blow cylinder 106 must make contact with a portion of preform 108 that is not immediately adjacent to blow mold 106 during a hot fill process. After the SBM process is performed on the blow mold 106-heated preform, cool air may be blown into the finished bottle through fine holes in stretch rod 110.
The SBM process may further include a cold set process. In a cold set process, a preform may be heated in an oven before being placed in the blow mold 106. Without the danger of damaging seals from a heated blow mold 106, as is the case during the hot fill process, it is possible to seat blow cylinder 100 on a portion of preform 108 adjacent to blow mold 106 or on blow mold 106 itself. After the SBM process has been performed on the oven-heated preform, blow mold 106 may be cooled provide a finished bottle.
Traditionally, SBM machines have included a separate blow cylinder for each of the hot fill and cold set operations. However, the practice of using two blow cylinders adds extra complication, expense, and process time to the SBM process.
There is a need in the art for a blow cylinder that is inexpensive and easy to operate, and capable of supporting fast SBM cycle times. There is a need for a blow cylinder that is simple, light weight, and able to actuate quickly with a relatively short braking distance. The braking must includes adequate damping to avoid damaging SBM machine components, performs, and end products, however. The present embodiments described below overcome these and other problems and an advance in the art is achieved.